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Lots of people have great success and cost savings by building plywood and glass tanks, especially the guys over at monsterfishkeepers, who typically keep fish so absurdly large that standard aquariums just don't cut it for them. If you're handy with power tools or just not scared of trying you should do a little googling for plywood aquariums.

what kind of tank are you building? glass, acrylic, plywood? any specific questions you have about the build? how much of it do you already have planned? will really help once we know what kind of materials you're going to use.

I helped a friend build an interesting tank. We used a wine barrel. Was a little complicated but in the end wow!! built a stand that looked like a X.
First difficulty getting the burnt charco out. Then the wood strips kept flexing so we fiber-glassed the inside to give it stability. Next we cut a large hole in the front. Then heated a large piece of plexi up and pushed from the inside through the cut out. It then kind of looked like a bubble coming out. Made sure to leave a lip on the outside of plexi so we could silicone from the inside to the wall. Cut the top off and put on hinges. So Cool every thing not in exact order of construction. Reinforcing around the cut-out so the boards didn't move took awhile.

one of my specific question is on how to join the glass ? will only silicon help such a huge tank ?

Always does me good to see someone doing their best to provide an arowana with decent housing.

Yes this can be done in glass as long as you use braces. I bought a used eheim from a guy who had a tank even longer than this all made out of glass. As said, monsterfishkeepers is probably the place to look for more information. And don't underestimate the weight of something this big. Water alone it will be 810kg. Let's say another 250-300kg for the glass.
What are you going to do to filter all that? A sump? That's easily another 200kg.